I've pulled the exhaust off my '91 K100RS 16V.
 As you can see in the pic, the flanges are badly corroded. Looks like flanges are permanently attached to the pipes on 16Vs. My plan now is to grind the rust away and either weld the flanges to the pipe or make up collets to hold the flange against the lip on the end of the pipe. I'd be grateful for some information about what they were like when new. Should the flanges 'float' on the pipe so they can be aligned with the studs or were they originally welded up solid? And are they just that bit of flat 5mm plate or did they incorporate a sleeve that's rusted onto the pipe on mine?


All the nuts were rusted on so the studs have come out. (and been thrown in the bin). As you can see in this pic, one stud on #2 brought an insert out with it. Looks a little diffrent from your run-of-the-mill helicoil. Do all the studs holes in the head have inserts or is this a repair?

https://www.ebay.co.uk/itm/BMW-K100-exhaust-headers-and-collars/153847943390?hash=item23d20d50de:g:daMAAOSw1T1eLbQc

I'd jump: that's a good price for decent set of exhaust pipes.

Pic.2: That looks like a timesert/nutsert. It's a repair, but unlike a helicoil it has a thread rolled down a solid tube. I'd try and wind it back in. Normally I reserve ***serts as a last-chance-saloon repair for the simple reason that (e.g. off the top of my head and IIRC) a 6mm helicoil requires a 6.2mm drill and 7.0mm tap, but a 6mm ****sert requires a 6.5mm drill and an 8mm tap. It takes a much bigger hole to fit.

Oh yes - you haven't pinged out the old gaskets yet as I can still see them. Highly recommended tool for doing just that:

https://www.ebay.co.uk/itm/Draper-Automotive-Universial-O-Ring-Oil-Seal-Puller-Remover-Tool-Hook-37439/302147640823?epid=1205673631&hash=item4659671df7:g:yksAAOSwHMJYNx67

Dai wrote:https://www.ebay.co.uk/itm/BMW-K100-exhaust-headers-and-collars/153847943390

I'd jump: that's a good price for decent set of exhaust pipes.
...

They're for an 8V. I suppose I could saw the header pipes off mine, leaving a length of straight pipe sticking out of the silencer, and sleeve the 8V headers on to them. Might stop them cracking at the welds too.

Something to think about. Thanks.

Nothing to stop you mounting a square silencer on the RS. You'll need to fiddle a bit around the mounting point, but it's not insurmountable. Otherwise, if you source another 16v pipe, fit a jubilee clip around all four pipes and about three inches forward of the welds. It shifts the resonant frequency of the exhaust and greatly reduces the probability of the welds cracking.

I quite like the square silencer now, has a certain quirky charm. Although I was rather rude about them 35 years ago. But I'd have to buy one of those too and this bike is in a mend and make do phase.

Is the only difference between 8v and 16V headers that the flange is loose on the 8V and welded in situ on the 16V? Or is there something more subtle going on?

And, on a more general point: the headers look to be quite different lengths going into the collector although they should ideally be the same length. They do bend all over the place but I don't think there's enough to equalise the lengths. Is this something the aftermarket exhausts try to address?

I'm thinking the newer exhaust is made to look smaller for looks. 

Another way of looking at it is if cylinder four header, for example, goes further into the muffler, it's overall length is more equal to the others, for a more balanced performance. I've noticed that on my bike too.

The two valve exhaust can fairly easily be fitted to a 16 valve K100.  Took me an afternoon, mostly fiddling with the centerstand and lining up the hanger with the mount.  No perceptible difference in performance or sound. 

A very minor loss of cornering clearance on left turns, but I ride like an old man, not wanting to scratch up the luggage in a low side.

If you do go for a square pipe, I'd leave the exhaust-to-silencer clamps off. They're a bastard to get lined up so that you can tighten them up individually and the inner one inevitably corrodes so badly that you can't unscrew it again. Low Flying Brick has no clamps on and there's no increase in noise, nor is there any trace of carbon around the joints.

Dai wrote:If you do go for a square pipe, I'd leave the exhaust-to-silencer clamps off. They're a bastard to get lined up so that you can tighten them up individually and the inner one inevitably corrodes so badly that you can't unscrew it again. Low Flying Brick has no clamps on and there's no increase in noise, nor is there any trace of carbon around the joints.

Any backfire issues?

None whatsoever.

Cobraexhaust have